1. Field of the Invention
The present invention relates to an image heating apparatus, such as a heating fixing apparatus, or the like, that is mounted in an image forming apparatus (such as a copier, a printer, or the like) adopting an electrophotographic method or an electrostatic recording method.
2. Description of the Related Art
Conventionally, a heating apparatus adopting a heat roller method or a film heating method has been widely used as an image heating apparatus, such as a heating fixing apparatus.
In contrast to a heating apparatus adopting a heat roller method, a heating apparatus adopting a film heating method is an on-demand and energy-saving apparatus in which electric power is not supplied during a standby state and power consumption can be minimized.
Such a heating apparatus includes a fixedly supported heater, a flexible member moving or rotating while contacting the heater, and a pressing member that forms a nip portion with the heater via the flexible member, and heats a material to be heated by heat from the heater via the flexible member while grasping and conveying the material between the flexible member and the pressing member at the nip portion.
More specifically, a ceramic heater is generally used as the heater. The ceramic heater has a basic configuration that includes an insulating, heat-conductive and low-heat-capacity ceramic substrate (heater substrate) made of alumina, or the like, and an electrically-heat-generating resistive layer made of silver-palladium (Ag/Pd), Ta2N, or the like formed on the substrate in the longitudinal direction of the substrate. The electrically-heat-generating resistive layer is heated by causing electric current to pass therein, so that the temperature of the entire heater including the substrate is rapidly raised. The temperature rise of the heater is detected by temperature detection means, such as a thermistor, or the like, and is fed back to a current control unit. The current control unit controls current supply to the electrically-heat-generating resistive layer so that the temperature of the heater detected by the temperature detection means is maintained at a predetermined substantially constant temperature (fixing temperature).
In order to efficiently transmit heat from the heater to a recording material, serving as a material to be heated, a thin heat-resistant and flexible resin film (hereinafter termed a “fixing film”) in the form of a cylindrical film or an endless film is generally used as the flexible member.
A heat-resistant and elastic pressing roller is generally used as the pressing member, and forms a nip portion (hereinafter termed a “fixing nip portion” or a “heating nip portion”) having a predetermined width (in the sheet feeding direction) by being in pressure contact with the heater via the fixing film, serving as the flexible member, against the elasticity of the pressing roller.
The fixing film, serving as the flexible member, is movably or rotatably driven by the rotation driving of the pressing roller, serving as the pressing member, or by driving member other than the pressing roller, to be conveyed and moved in tight sliding contact with the surface of the heater at the fixing nip portion.
In a state in which current is supplied to the electrically-heat-generating resistive layer of the heater, the moving driving or the rotation driving of the fixing film is started, the heater is subjected to temperature control after the temperature of the heater is raised to a predetermined fixing temperature, and the conveying/moving speed of the fixing film is stabilized after being increased to a predetermined speed, a recording material, serving as a material to be heated, having an unfixed image formed thereon is guided between the fixing film and the pressing roller at the fixing nip portion. The recording material is heated by heat from the heater via the fixing film while being grasped and conveyed between the fixing film and the pressing roller at the fixing nip portion, so that the unfixed image is heated and fixed on the surface of the recording material. A portion of the recording material passing through the fixing nip portion is conveying by being separated from the surface of the fixing film.
In the above-described heating apparatus, as a safety countermeasure during thermal runaway of the heater, i.e., when the heater becomes in an excessive high temperature state as a result of continuous current supply to the electrically-heat-generating resistive layer of the heater because current supply to the electrically-heat-generating resistive layer becomes in an uncontrolled state due to a some failure, a safety element (hereinafter termed a “thermoprotector”), such as a temperature fuse, a thermoswitch, or the like, for forcedly shutting down current supply to the electrically-heat-generating resistive film by detecting an excessive high temperature higher than a temperature allowed for the heater is disposed so as to contact a surface of the heater opposite to the sliding surface with the fixing film.
The thermoprotector is disposed so that a heat collecting plate thereof contacts a surface of the heater opposite to a sliding surface with the fixing film. In this case, in order to prevent uneven heat collection of the heat collecting plate, a heat-conductive grease is coated on a contact surface between the heat collecting plate of the thermoprotector and the heater.
Since the thermoprotector has a relatively large heat capacity, heat quantity generated in the electrically-heat-generating resistive layer is transferred to the thermoprotector at the contact position of the heater with the thermoprotector. As a result, sufficient heat quantity is not supplied to the recording material as compared to positions of the heater other than the contact position of the heater with the thermoprotetor, sometimes resulting in a failure in fixing at the contact position. In order to prevent such a phenomenon, heat quantity at the contact position is secured by increasing the resistance value of a portion of the electrically-heat-generating resistive film corresponding to the contact position by more or less decreasing the width of that portion. Thus, the amount of heat supply to the recording material is made uniform (or to have a designed temperature distribution) over the longitudinal direction of the heater (a direction orthogonal to the sheet feeding direction), to realize excellent heating fixing not having unevenness in fixing.
It has become clear that if the contact position of the safety element with the heater deviates from a designed position in a direction of ends of the heater (the sheet feeding direction), a high-temperature offset of a toner image and a failure in an image, such as unevenness in gloss, or the like, occur.
Since the safety element is mounted on the heater using many components, such as a heater holder, a safety-element holder, and the like, accuracy in mounting of the safety element results from addition of dimensional tolerances of the respective components. Accordingly, the contact position of the safety element with the heater tends to vary with respect to the designed position. It has become clear that the temperature distribution in a contact portion of the heater with the safety element differs from the temperature distribution in other portions of the heater in the longitudinal direction, and a high-temperature offset, a failure in fixing, and unevenness in gloss occur at the contact portion of the heater with the safety element.